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New pest may cause damage to area soybeansPosted Monday, August 30, 2010, at 4:58 PM
It seems there is always some new organism that likes to feed off your crop. An increase in lodging in your soybeans may be the result of the Dectes stem borer. Dectes stem borer is an insect that tunnels in soybean plants and has been found in significant numbers in soybean fields surveyed in 12 counties in southeast Missouri. Surveys by MU entomologists found stem borers in low numbers in major soybean counties along the Missouri river from St. Charles to St. Joseph and as far north as Dekalb and Lewis county in northern Missouri.
Kelly Tindall, entomologist at the University of Missouri Delta Center indicated that the economic impact of the stem borer is unknown. Sometimes the Dectes causes the plants to fall over and sometimes not. Confounding the issue of damage is research that shows some soybean plants, with only one larva in a plant stem, produce more seed. However, the survey found up to six larvae in some plants which are likely to lower yields according to Kelly. The Dectes stem borer is native to the United States and traditional host plants include weeds such as cocklebur, wild sunflower and both common and giant ragweed. Soybean fields that had these weeds had more larvae in the weeds than in the soybean plants. This insect also is found in neighboring states.
The adult Dectes stem borer is gray, ranging from dark to light. The beetles are about ¾-inch long. Black and white antennae longer than the insect's body give the common name of longhorn beetle. Larvae have an "accordion-like" appearance with an orange-red head and reach about ½ to 5/8 inch in length when they are fully mature. After hatching, the larvae feed on the outer stem before they bore into the main stem. The larvae tunnels within the main stem and feeds on the pith tissue. In late summer, the larvae move to the base of the plant and girdle the interior of the stem approximately 2 inches above the soil surface. The larvae then plug the stem with their frass, creating a cell where they overwinter. Damage from this insect may coincide with increased no-till soybean plantings, which provide undisturbed overwintering sites for the larval stage. The presence of their weed hosts in and around the soybean fields can further increase the risk of Dectes stem borer infestations. The greatest losses often occur in early maturing full season soybeans, early planted fields, areas planted to continuous soybeans and weedy fields (cocklebur and ragweed are alternate hosts).
Scouting procedures are difficult and limited. At this time of year, probably the best indication of Dectes damage is to look for lodged plants. Cut open the stem and look for the presence of the larva and its feeding damage. Counts of larva damage and lodged plants will be useful in determining management strategies.
Several practices can help reduce stand and yield losses. 1. Control weed hosts. 2. Harvest the crop as soon as possible to reduce losses from lodging. 3. Fall tillage will reduce overwintering survival of larvae. 4. Avoid planting soybeans in fields adjacent to fields that were heavily infested the previous year.
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As our local University of Missouri Agronomy Extension Specialist, Crook has been writing a column for the print edition Agriculture page for the past three years and we will now be sharing it on our web version. Crook has a bachelor and masters degree in agronomy from University of Missouri and received his doctorate in Agronomy from Kansas State University. He was in soybean variety development research for 22 years for various seed companies and has been Saline County's agronomy specialist for 10 years.